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Grease lubrication

The values for the adjustment factor for the bearing load (fP) provided in diagram 1, diagram 2, diagram 3 and diagram 4 are valid for grease lubrication. When greases with a base oil viscosity between 100 and 200 mm2/s at 40 °C (105 °F) are used, the reference speed does not need to be adjusted (fν = 1). For other base oil viscosities, the value for fn needs to be compared with the value for ISO VG 150 oil. The adjusted reference speed for grease lubrication can be estimated using

nar = nr fp (fν actual base oil viscosity / fν ISO VG150)

Example 1

An SKF Explorer 6210 deep groove ball bearing is lubricated via an oil bath. The oil viscosity is 68 mm2/s at 40 °C (105 °F), load P = 0,24 C0. What is the adjusted reference speed?
For bearing 6210: dm = 0,5 (50 + 90) = 70 mm and nr = 15 000 r/min. From diagram 1, with dm = 70 mm and P/C0 = 0,24, fP = 0,63 and with P/C0 = 0,24 and ISO VG 68, fν = 0,85.

nar = 15 000 x 0,63 x 0,85 = 8 030 r/min
The limiting speed for this bearing is 10 000 r/min, which is higher than its adjusted reference speed. In cases like this, the lower of the two rated speeds is the most significant for the service life of the bearing. In general, speeds up to the adjusted reference speed provide conditions favourable for extended bearing service life. Higher speeds up to the bearing limiting speed can, in principle, be adopted after further assessment of the specific temperature conditions surrounding the bearing arrangement.

Example 2

A grease lubricated SKF Explorer 22222 E spherical roller bearing is subjected to a load P = 0,15 C0. The grease has a base oil viscosity of 220 mm2/s at 40 °C (105 °F). What is the adjusted reference speed?
For bearing 22222 E: dm = 0,5 (110 + 200) = 155 mm and nr = 3 000 r/min. From diagram 2, with dm = 155 mm and P/C0 = 0,15, fP = 0,53 and with P/C0 = 0,15 and ISO VG 220, fν actual = 0,83; with P/C0 = 0,15 and ISO VG 150, fν ISO VG150 = 0,87.

nar = 3 000 + 0,53 + 0,83/0,87 = 1 520 r/min
Up to this speed, the bearing thermal conditions are generally considered normal. Higher speeds, up to the limiting speed of 4 000 r/min, can also be considered under certain circumstances. This would require an assessment of the temperature rise of the application based on the actual frictional and cooling conditions within the application. In cases like this, the lubricant, heat dissipation, bearing internal clearance and accuracy of the bearing seats must be verified and adapted to accommodate higher speeds. For these types of evaluations, contact the SKF application engineering service.
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